a) Field of the Invention
The invention is directed to an autofocusing device for an optical instrument, in particular for a microscope.
b) Description of the Related Art
Autofocusing devices are used where there is a need for bringing an object that is to be observed or to be examined into a position that is as precise as possible relative to the observation instrument, in particular into the focal point of the observation instrument. A large number of commonly known autofocusing devices use their own illumination source, the light of which is directed onto the object and evaluated after interaction with the object for the purposes of determining a distance or a deviation from a reference position. When the distance or the deviation from the reference position is known, an automatic position correction can be carried out.
From the state of the art, autofocusing devices for optical instruments are known which essentially differ with regards to the following performance parameters:
Resolution in direction of the optical axis (subsequently referred to as the z-axis),
depth of the capture or working range,
whether generating a directional signal for a correcting adjusting movement is possible,
attainable measuring speed.
The triangulation methods often used for determining distances permit a relatively large capture range, but they are limited to values to the order of approximately 300 nm with regards to the resolution along the z-axis, and therefore unsuitable for the optical inspection of semiconductor components (wafers), since for these resolutions to values to the order of approximately 50 nm with a capture range of a several micrometers are necessary.
Autofocusing devices which are, for example, used in CD-players have a relatively large capture range and furthermore attain a high z-resolution, but only if the surface to be measured has very good reflective properties, since otherwise the z-resolution in particular drops off sharply.
The aforementioned autofocusing devices usually direct laser light onto the object that is to be examined, but if the wavelength spectrum of the main system differs a lot from the autofocusing system, systematic focusing errors result which amongst other things depend on the material properties and the microstructure, for example a surface coating, of the object to be examined.
Based on this, it is the primary object of the invention to create an autofocusing device which permits high focusing precision and high focusing speed combined with simple construction.
For an autofocusing device for which the illuminating light is directed through imaging optics onto an object moving in a direction at least approximately vertical to the optical axis of the imaging optics, this object is met by arranging a diaphragm device in the illumination ray path between illumination source and imaging optics with at least one diaphragm opening which extends in a direction aligned with the direction of movement of the object; by arranging a receiving device in the measuring light ray path for the measuring light coming from a measuring location on the object, which receiving device has separate reception areas that can be evaluated individually and that are arranged in a row beside each other in a direction aligned with the direction of movement of the object; by inclining the diaphragm opening relative to the optical axis of the illumination ray path or the receiving areas arranged in a row beside each other relative to the optical axis of the measuring ray path by an angle such that 0 degrees less than xcex1 less than 90 degrees by means of which the image of the diaphragm opening is inclined relative to the receiving areas while the receiving device and the diaphragm opening are positioned relative to each other in such a way that characteristic measuring values are measured by the receiving areas when the measuring location is in or near the in-focus position; by the presence of a synchronous control which initializes the sequential reading of the measuring results in the receiving areas, wherein at the time (t1) of read-out the receiving device (e1) the measuring location is in a position (p1), at the time (t2) of read-out the receiving device (e2) the measuring location is in a position (p2) and so forth, and an evaluating device is provided which compares the measured values read from the receiving areas with desired values and from these generates signals for the optical instrument.
Preferably, the evaluation device is to be designed for the generation of adjusting signals for an adjusting device by means of which the direction of movement of the object into the focal plane of the imaging optics is initialized if there is a deviation between the read out measuring values and the stored desired values.
The imaging optics can nonetheless also be designed for the determination of other command signals for the optical instrument, for example for the activation of an image recording device and similar things.
Because of the inclination of the diaphragm device relative to the optical axis of the illuminating light or the inclination of the receiving device relative to the optical axis of the measuring light, an intensity distribution results on the receiving device which is aligned with the direction of movement of the object and which is characteristic for the position of the measuring location relative to the imaging optics.
The path of movement of the measuring location intersects with an imaginary plane the location of which is determined by the fact that if a mirror were to be inserted into the optical system in this location it would effect an optical conjugation of areas on the diaphragm device with assigned receiving areas and that the characteristic measuring values can be read out when the moving measuring location touches this imaginary plane.
By means of this, the autofocusing device according to the invention permits multiple measurements of the same measuring location on one object by means of one slit aperture or of a row of individual diaphragms, so that a set of data is available for the evaluation of the focal position. Such multiple measurements are particularly advantageous if the surface structure of the objects to be examined is highly fissured.
Advantageously, a device for the continuous advance of one or more objects can be present, which device is coupled to a synchronous control, wherein the direction of advance should be at right angles to the optical axis of the imaging optics.
The arrangement according to the invention described so far is more suited for the gathering of measuring values relating to the focal position of a moving object or of a measuring location; such an operating mode, where the imaging optics and the object are moving relative to each other, is here to be referred to as a xe2x80x9cdynamicxe2x80x9d operating mode, wherein the movement can, for example, be continuous or also a stepped advance movement of the object.
For a xe2x80x9cstaticxe2x80x9d operating mode, on the other hand, where the image information is to be gained about measuring locations on objects which are at rest relative to the imaging optics, the following arrangement according to the invention is better suited.
For this autofocusing device, the illuminating light is directed through imaging optics onto the surface of an object at rest; a diaphragm device is provided in the ray path between the illumination source and the imaging optics which has at least one diaphragm opening and extends in a preferred direction V; a receiving device for the measuring light coming from the measuring object is present which has receiving areas arranged in a row in a direction Vxe2x80x2 corresponding to the preferred direction V and which can be evaluated individually; furthermore, the diaphragm opening and the optical axis of the illumination ray path or the receiving areas arranged in a row and the optical axis of the measuring ray path form an angle such that 0 degrees less than xcex1 less than 90 degrees, so that the image of the diaphragm device is incident on the receiving area at an inclination of the angle xcex1 and measuring light is incident on one of the receiving areas at maximum intensity when an assigned partial area of the diaphragm opening is in conjugation with this receiving area; an adjusting device for changing the distance of the object from the optical axis is present, and an evaluating device is provided which outputs information about having reached the focal position when during the course of the change of distance the measuring light causes measuring values on the receiving areas which correspond to preset comparative values.
Here also, the characteristic measuring values can be read when a measuring location is on an imaginary plane the location of which is determined by the fact that if a mirror were to be inserted into the optical system in this location an optical conjugation of areas on the diaphragm device with assigned receiving areas would be effected.
In this way, a change of position of a chosen measuring location relative to the focal point of the imaging optics can be initialized until this measuring location has reached the focal position.
This autofocusing device can furthermore be designed in such a way that the autofocusing device first determines that receiving area on which at a current distance z of the object surface from the imaging optics the maximal measuring light intensity is incident. After this, the difference is determined between the position of this receiving area and the position of a predetermined receiving area on which the maximal light intensity is only incident if the distance z corresponds to the focus distance. From this difference, a correcting variable is generated which corresponds to a distance for the adjusting movement.
If the adjusting device is, for example, equipped with an electric motor as a drive, a change of the distance z can be caused via the correcting variable until the maximal measuring light intensity impinges on the predetermined receiving area and the focus position is therefore reached and focusing is completed.
The receiving device can be realized in the autofocusing device for the xe2x80x9cdynamicxe2x80x9d as well as the xe2x80x9cstaticxe2x80x9d operating mode as a row of receivers which consists of a large number of individual sensors arranged in a row beside each other, wherein each receiving area corresponds to one individual sensor. In this case, the diaphragm device can advantageously be designed and arranged in the ray path in such a manner that it creates an xe2x80x9cillumination linexe2x80x9d which is incident lengthwise on the receiver row and overlaps it.
It is essential for the function of the arrangement according to the invention that either the diaphragm opening or the receiving device form an angle such that 0 degrees less than xcex1 less than 90 degrees with the respective optical axis of the ray path into which they have been inserted. It can also be envisaged that the diaphragm device as well as the receiving device are to be inclined against the ray path, but this requires predetermining differing angles of inclination for the image of the diaphragm device as well as the image of the receiving device.
A realization has, for example, proven itself for which the diaphragm opening is aligned vertical to the optical axis of the illumination ray path while the row of receivers forms an angle xcex1=45 degrees with the optical axis of the measuring ray path.
The diaphragm opening can, for example, be realized as a slit opening, wherein each individual sensor is assigned to one section of the slit opening. As an alternative, the diaphragm opening can also be realized as a pinhole diaphragm with a large number of individual pinholes arranged in a row beside each other, wherein, for example, one individual diaphragm is assigned as a partial area to each individual sensor.
A one dimensional CCD-array can be provided as the row of receivers, wherein one or more pixels of the one dimensional CCD-array respectively correspond to one individual sensor or one receiving area.
With the autofocusing device in the embodiment forms described so far, resolutions in direction of focusing with values to the order of 50 nm can be achieved for applications in connection with microscopes. Here, a capture range of a size to the order of several micrometers within which a meaningful signal is captured is possible.
For realizations of the autofocusing device which are adapted to particular applications, the diaphragm device can have a number of parallel slit openings or rows of individual diaphragms and by this generate a number of xe2x80x9cillumination linesxe2x80x9d incident on the receiving device. For these cases, the receiving device can have a number of rows of receivers aligned parallel to each other, wherein againxe2x80x94as has already been describedxe2x80x94one individual sensor can be assigned to a partial area of the slit openings or to one of the individual diaphragms for the purpose of evaluation.
It can also be envisaged that the receiving device is realized as an area position-sensitive detector and that each partial area of the area detector is optically assigned to a slit opening or an individual pinhole.
In principle, it is possible to use the measured intensities or brightness values directly for evaluating the focus position and to gain a signal from this with which the maximum of the intensity distribution can be determined, but it can also be envisaged that local contrast values instead of local intensities are measured and that further processing is based upon these.
The autofocusing device according to the invention can, for example, be implemented as a separate optical system in a microscope, wherein the optical system for the autofocusing device and the main system of the microscope are realized as separate subassemblies.
It is more advantageous, though, to share the use of functionally essential optical subassemblies for the autofocusing device as well as the microscope ray path by, for example, arranging the diaphragm of the autofocusing device in the illumination ray path between illumination source and imaging optics of a microscope, that is, roughly in the marginal area of an luminous field diaphragm. In this case, there is a multiple use of illumination source and imaging optics, by means of which a reduction of the number of system components and in particular a compact construction can be achieved.
The shared use of an illumination source has the added advantage that the autofocusing device is operating in the same wavelength range as the optical main system. This avoids systematic errors based on wavelength differences and therefore guarantees particularly high focusing precision.
The invention is subsequently to be explained by an embodiment example.